Blasting circuit wiring includes the firing or leading lines, any interconnecting wire and the legwires which are attached to the electric blasting caps. All may be properly connected to form an appropriate electric blasting circuit. To fire the blast an initiating current is introduced into the circuit from a firing source such as a capacitor discharge blasting device or any other approved source of electrical power.
The adapter of this invention is intended to act as a safety device for use when blasting near high voltage transmission lines.
The adapter may also be employed as a fast-acting circuit severer to interrupt electric current in configurations of electric blasting circuitry in which hazards to personnel could exist as the result of arcing malfunctions.
In commercial electric blasting near high voltage transmission lines the main hazard to personnel lies in the danger of suffering burns or electrocution if a blasting circuit wire is thrown into contact with an energized power line. One of the precautionary measures in use involves taping an explosive charge or detonating cord to the firing line to be severed. The explosive so taped is initiated by a millisecond delay electric blasting cap which is properly connected and electrically part of the electric blasting circuit. A typical millisecond delay electric blasting cap to be used in this application is the Atlas Rockmaster 1 which will detonate in about 8 milliseconds after the electrical current from a blasting machine is supplied to the blasting circuit. The detonation of the blasting cap initiates the explosive charge which severs the electrical firing lines which lead back to the terminals of the blasting machine. This prompt severing of the firing lines at some safe distance from the overhead transmission lines will insure that the person operating the blasting machine will not be electrically connected to the high voltage lines in the event that a wire of the blasting circuit is flung into contact with the power lines by force of the blast.
With the adapters disclosed in this invention the need for an explosive charge, in addition to an appropriate millisecond delay blasting cap, is eliminated. Instead, the line-severing adapter is affixed to the lines to be separated and the appropriate millisecond delay blasting cap is inserted into the adapter. When the firing current is applied to the blasting circuit the millisecond delay cap will function after the short delay interval to sever the firing lines before any other circuit wiring could reach the overhead lines.
In shaft sinking and tunnel driving operations a blasting circuit of electric blasting caps connected in parallel or reverse parallel electric configuration is widely preferred. These circuits are most often energized by current switched to the circuit directly from a 220 or 440 volt power line. A wide range of delayed periods extending to 71/2 seconds, and beyond, are often used in the parallel circuit to achieve the desired sequence of rock-throwing in a given blast. If the power line firing switch is closed to energize the blasting circuit, the continued application of current to the circuit could result in electric arcing within the caps. This arcing, with the attendant generation of intense heat, can cause arcing malfunctioning, that is, rupture of the cap, blowout of the cap sealing plug, expansion of the cap body with flame bypassing the delay element or accelerated burning of the pyrotechnic delay mixture.
Any of these effects can result in seriously erratic delay timing. Some can cause an extinguishing of the burning delay mixtures. Others can produce a premature detonation of the explosive in which the cap is embedded. A most serious effect results when the cap fails to detonate and the intense arc ignites the explosive to cause a slow burning "hangfire." This latter effect has been known to result in blasts taking place up to a half hour after the main blast took place which of course presents a hazard to personnel reentering the area.
To prevent an arcing malfunction the firing line severing adapater of the present invention would be affixed to one of the two firing lines, preferably to one of the temporary firing lines near the blast site. An appropriate delay cap, such as an Atlas Rockmaster 2 would be inserted into the adapter to sever the firing line approximately 25 milliseconds after the current was applied to the blasting circuit. As a doubly safe measure, a line severing adapter could be affixed to each of the two firing lines, just ahead of the main blasting circuit at the blasting site.
Other lead severing arrangements are known which act to sever the blasting circuit leads through the explosive force emanating from a blast hole. Such devices are illustrated by the Parr U.S. Pat. No. 2,607,442 issued in 1946. In addition, deflecting or catching devices are known which operate to prevent cap wires from flying into contact with high voltage transmission lines, personnel or other electrical operating equipment. Such a deflection device is illustrated by the Rodriquez U.S. Pat. No. 3,339,486 issued in 1967.
Also it is known to use fast acting electrical circuit breakers between the firing switch and the blasting circuit firing line in the parallel or reverse parallel electrical configuration described above in shaft-sinking and tunnel-driving operations.